A conventional sheet stacking device for stacking sheets having images formed thereon includes a stacking tray on which sheets are stacked, and an auxiliary tray for extending the stacking surface of the stacking tray.
Patent Literature (PTL) 1 describes a sheet stacking device that includes an auxiliary tray pivotably supported by a stacking tray. As illustrated in FIG. 11, an auxiliary tray 412 described in PTL 1 is disposed to be pivotable about a pivot point O8 between a storage position at which the auxiliary tray 412 is stored in a stacking tray 411 and an extension position at which the auxiliary tray 412 extends the length of a sheet stacking surface of the stacking tray 411 in a sheet discharging direction.